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The proton drip line is the proton-rich boundary at which one- or two-proton emission becomes energetically possible. It is governed by proton separation energies, rather than the energetics of beta decay alone.

With bare nuclear masses, the proton separation energy is the energy needed to remove a proton. A negative value means the daughter plus proton has lower rest energy. Proton emission can still have a finite lifetime because of a Coulomb or centrifugal barrier.